1. Field of the Invention
This invention relates to an imaging system and particularly, although not exclusively, to a thermal imaging camera.
2. Description of the Related Art
Imaging systems are known where radiant energy which may be visible light, or infrared, or in the X-ray spectrum, for example, is converted into electrical signals. The term “electrical signals” used herein is meant in its broadest aspect to include electromagnetic and opto-electric signals. Such energy also includes thermal energy used for viewing scenes where extreme temperature differences are evident, for example in fire fighting, where thermal energy is converted by a detector into electrical signals for viewing by an operator. Although this invention is intended principally in thermal imaging situations, the invention is applicable to imaging systems where a compromise is found between dynamic range and low contrast sensitivity.
FIG. 1 shows in block schematic form the basic components of a thermal imaging camera. Referring to FIG. 1, a lens 1 directs thermal radiant energy to a detector 2. The detector 2 may be of any known type and normally currently comprises a two-dimensional planar bolometric array which converts changes in temperature to a change in resistance or polarisation. Bolometric detectors converting temperature to a change in resistance include detectors having a vanadium oxide layer made, for example, by Infraredvision Technology Corp., Type No. ITC-1000, or where the detector is amorphous silicon and such a detector is made by Ulis, under Type No. UL 01011. Detectors of the polarisation type are made, for example, by Raytheon Commercial Infrared, under Type No. 200D.
The detector comprises a plurality of pixels which are typically scanned pixel by pixel and line by line. The output from each pixel is electronically processed by processing electronic circuitry 3, and such circuitry is known from ISG Thermal Systems Limited Talisman Elite thermal imaging camera, Part No. K82MAD002CBAB, before being passed to a display 4, which is typically an LCD display, although it will be understood that the type of display utilised is application dependent. The display may be incorporated with the lens detector and processing electronics in a portable handheld unit or the display may be remote from the lens detector and processing electronics.
In operation, the thermal imaging camera detector 2 receives thermal radiation and the detector converts the thermal energy to electrical signals. The sensitivity of the detector is the relationship between the difference in thermal energy received by the detector and the magnitude of the difference in the resulting electrical signal output. It is known to adjust the sensitivity of the detector, either by adjusting a bias voltage applied to the detector between predetermined levels, or by adjusting the integration time by means of adjusting the duty cycle of a digital control signal.
The sensitivity of the detector arising from exposure to thermal radiation directly relates to the range of thermal energy that the detector can convert into electrical signals before the detector becomes saturated. Thus, if the detector is biased for high sensitivity, then it is able to convert the scenes with a low temperature difference into electrical signals with a difference sufficiently large enough to be processed effectively by the processing electronics 3. However, the higher the sensitivity, the lower the maximum temperature before the detector saturates. In other words, if the detector is biased for low sensitivity to avoid saturation, then only a scene with a larger temperature difference can be effectively converted into electrical signals to be processed. In a thermal imaging camera, this biasing arrangement is directly related to the sensitivity of the detector. The greater the sensitivity, the lower the scene contrast that can be effectively converted.
Alternatively, the sensitivity may be controlled via integration time as follows. The length of time that the detector accumulates electrical signals arising from exposure to thermal radiation, directly relates to the range of thermal energy that the detector can convert into electrical signals before the detector becomes saturated. Thus, if the detector accumulates for long periods, then it is able to convert the scenes with a low temperature difference into electrical signals with a difference sufficiently large enough to be processed effectively by the processing electronics 3. However, the longer the accumulation, the lower the maximum temperature before the detector saturates. In other words, if the detector accumulates for short periods of time to avoid saturation, then only a scene with a larger temperature difference can be effectively converted into electrical signals to be processed. In a thermal imaging camera, this accumulation period is called the integration time and is directly related to the sensitivity of the detector. The longer the integration time, the lower the scene contrast that can be effectively converted.
However, in such known systems, an operator is faced with using one or other sensitivity level. Thus, consider a fire fighter viewing a scene containing a high heat source, e.g. a fire. Ideally, the fire fighter needs to be able to recognise two pieces of information using their thermal imaging camera, namely:
detail around the fire that is cooler where a body or explosive substance may be located, and
detail within the fire that is hot where information about the source and nature of the fire may be located.
So as to view the detail around the fire, the integration time, and hence the sensitivity of the detector, should be set at a maximum. This allows low contrast, low level information to be converted into electrical signals that may be viewed by a fire fighter. However, the fire in the scene will be saturated and detail within the fire will be lost. This situation is shown in the accompanying FIG. 2 which shows a high contrast scene with long integration and high sensitivity. The alternative is to reduce the integration time, thereby reducing the detector sensitivity, to provide good contrast within the fire. However, low contrast, cooler background detail is lost. Such a situation is shown in the accompanying FIG. 3 which shows the same scene as FIG. 2 with short integration, low sensitivity. In both FIGS. 2 and 3 the fire is represented by a very hot ceramic toroidal disc that is located on the right of the display in each FIG. 2, 3.
Current thermal imaging cameras switch between integration/sensitivity modes in order to view large thermal differences in a scene. Such switching is often performed automatically as a fire fighter pans a camera around a scene. However, when a hot part of the scene is viewed and the camera switches to low sensitivity mode, then details in the cooler parts of the scene may be lost, as indicated in FIG. 3. Thermal imaging cameras normally operate with a maximum integration period, i.e. high sensitivity which is termed herein, for convenience, the “non-EI” mode (NEI), and upon detection of thermal energy above a predefined threshold, so the integration time is switched to a minimum or minimum sensitivity mode which is termed herein, for convenience, the “EI” mode. As indicated above, such switching enables high level data to be viewed by altering the dynamic range of the detector. Such switching takes place over a period of several fields, typically five or six fields, during which time a scene is not updated. Clearly, such loss of information is detrimental.
For the sake of candour in providing prior art disclosure, the inventors are aware that instead of varying the integration time, it is also known to adjust the bias voltage applied to a detector to effect high or low sensitivity detection.
As will be seen from FIGS. 2 and 3, the disadvantage of the prior art is the loss of detail in low contrast areas of a viewed scene and the interruption of viewing caused by the switching of sensitivity levels.
Additional disadvantages include the potential to confuse a user by the necessity to display different transfer functions between scene temperature and display intensity at different times.
The present invention seeks to at least partially mitigate the foregoing disadvantages.